The present invention is concerned with water-based fluids employed in the drilling of oil, gas and other wells such as geothermal wells. The invention is also concerned with an additive for such drilling fluids and with methods for drilling wells employing such fluids.
Water-based drilling fluids, often called "drilling muds" since many comprise clay particles in aqueous dispersion, are commonly employed as follows. The fluid is pumped down a hollow drill pipe and, at high velocity and high shear (more than 500 sec.sup.-1), through the orifices or "jets" of the drill bit located at the bottom of the drill pipe. In this way the fluid cools and lubricates the drill bit and rapidly removes rock cuttings made by the cutting action of the bit. To accomplish this task, the fluid should ideally have little or no resistance to flow, i.e. low viscosity. The drilling fluid must then carry out of the bore-hole the rock cuttings and other solids incorporated into the drilling fluid, e.g. weighting materials such as barite added to the drilling fluid to increase its density. This is done by pumping the fluid back to the well surface at lower velocities and lower shear rates (less than 200 sec.sup.-1) through the "annulus," the hole outside the drill pipe made by the drill bit. To prevent cuttings, weighting materials, and other solids from slipping back down the annulus during both drilling and drilling interruption periods, the fluid should have a relatively high viscosity during this phase of the drilling fluid circulation. Such fluids which exhibit low viscosities under high shear and high viscosities under low shear are commonly called "pseudoplastic" fluids.
In addition, because downhole temperatures may exceed 150.degree. F., e.g. may range from 150.degree. to 400.degree. F. (65.degree. to 204.degree. C.), the fluid's components should not degrade on exposure to these higher temperatures. The latter is particularly important during the drilling phase where successful transport of cuttings and other solids up the annulus depends on a fluid's capacity to yield high viscosity at low shear following exposure to high downhole temperatures. It should be noted that the drilling fluid cools significantly during its return trip to the surface as well as on reaching surface "mud pits" or fluid storage tanks. This permits the use of fluids which may lose most of their viscosity on brief exposure to high downhole temperatures provided substantial viscosity is regained on the return trip to the surface. At the surface, cuttings are removed, the drilling mud is cooled to near-ambient temperature and additional additives are mixed into the mud to restore the mud to initial properties.
Many drilling fluids have been described in the art. For instance, U.S. Pat. No. 4,322,301 describes a water-based drilling fluid which is used at high drilling temperatures occurring during deep drilling.
In general, currently used drilling fluids which employ materials other than clays as viscosifiers lose their low shear viscosity imparting properties after brief exposure to temperatures of about 300.degree. F. (about 150.degree. C.) or higher. Such high temperatures are commonly found in deep wells where weighting additives are needed to prevent the influx of downhole formation water into the drilling fluid as well as in more shallow hot wells, such as geothermal wells. In general, these same drilling fluids do not possess sufficient low shear viscosity to adequately suspend moderate levels of weighting materials (e.g. 12 lb/gal barite) in surface tanks where mean shear rates are substantially reduced (e.g. to 1-10 sec.sup.-1) and there is little "forward movement" of the fluid to help maintain high density weighting materials like barite in suspension.
It is an object of the invention to provide a pseudoplastic drilling fluid having increased thermostability as well as increased capacity to suspend solids present during both shallow and deep well drilling. It is a further object of the invention to provide a drilling fluid with increased capacity to suspend high density weighting materials while said fluid is being held in a surface mud pit or storage tank.
U.S. Pat. No. 3,557,016 describes a composition comprising Xanthomonas hydrophilic colloid and locust bean gum. The patent discloses a medium pyruvate xanthan containing medium amounts of pyruvic acid of less than 5 percent by weight. The composition disclosed forms a gelatinous product with water and is utilized in the production of a variety of food products. Application of the composition in well drilling is not disclosed.